1. Field of the Invention
The present invention relates to band pass filter devices and duplexers including the same.
2. Description of the Related Art
To date, various band pass filters have been used in mobile communication apparatuses such as cellular phones. In cellular phones, for example, frequency bands separating the pass bands of a plurality of channels have become narrower. Hence, it is strongly required that attenuation in the vicinity of the pass bands be increased.
Japanese Unexamined Patent Application Publication No. 62-261211 discloses a filter device in which a delay element is connected in parallel with a main filter. The characteristics of the delay element are set in such a manner that, at a desired frequency within the attenuated region of the main filter, the delay element has nearly the same amplitude characteristics as the main filter and has a phase which is different from that of the main filter by (2n−1)π (n is a positive integer). As a result, a direct wave at the desired frequency is cancelled out, and the attenuation at the desired frequency can be increased.
In Japanese Unexamined Patent Application Publication No. 62-261211, the delay element described above is formed of, for example, a transversal SAW filter.
On the other hand, Japanese Unexamined Patent Application Publication No. 10-256809 discloses a filter device in which an auxiliary circuit is connected in parallel with a band pass filter. In the auxiliary circuit, a variable attenuator is connected in series with a variable phase shifter. The phase can be adjusted by the variable phase shifter. In the variable attenuator, the attenuation of a signal propagating through the auxiliary circuit can be adjusted.
As disclosed in Japanese Unexamined Patent Application Publication No. 62-261211, when a signal flowing through a circuit that is connected in parallel with the filter and a signal flowing through the main filter have the same amplitude but opposite phases at a desired frequency, the attenuation can be made high at the desired frequency. Further, when the filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-256809 is used, by adjusting the adjustment amounts of the variable phase shifter and variable attenuation unit and the attenuation in the variable attenuation unit, the above-described condition, i.e., the same amplitude but opposite phases is satisfied.
However, active elements such as an IC and a diode are used in the auxiliary circuit including the variable phase shifter and variable attenuator disclosed in Japanese Unexamined Patent Application Publication No. 10-256809. Hence, a control circuit is required, thereby making reductions in cost and size difficult. In addition, control currents for operating the active elements are needed, leading to the problem of high power consumption.